Blockchain-Aided Digital Twin Offloading Mechanism in Space-Air-Ground Networks

Space-air-ground (SAG) integrated heterogenous networks can provide pervasive intelligence services for various ground users (GUs). The network can help cellular networks release network resources and alleviate congestion pressure. Moreover, one important application of the network is that digital twin (DT) can enable nearly-instant wireless connectivity and highly-reliable data mapping from physical systems to digital world in a real-time fashion. The integration of SAG and DT (SAG-DT) reduces the gap between data analysis and physical status, which can further realize robust edge intelligence services. However, the random computation task arrival, time-varying channel gains, and the lack of mutual trust among ground GUs hinder better quality of service in the promising SAG-DT network. In this paper, we envision a SAG-DT integrated blockchain model to transfer the task data to the aerial network, and then perform the computation offloading, energy harvesting and privacy protection. Moreover, we propose a Lyapunov-aided multi-agent deep federated reinforcement learning (MADFRL) algorithm framework to optimize the CPU cycle frequency, the size of block, the number of DTs, and harvested energy to minimize the execution costs and privacy overhead. Extensive performance analyses indicate that the MADFRL algorithm framework can strengthen the data privacy via blockchain verification mechanism and approaches the optimal performance on the basis of lower computation complexity. Finally, simulation results corroborate that the proposed Lyapunov-aided MADFRL algorithm is superior to advanced benchmarks in terms of execution costs, task processing quantities and privacy overhead.

I. Introduction

In the last few years, terrestrial cellular networks face huge bottlenecks, such as traffic congestion [1], low coverage, high construction costs, large power consumption, and so on, which cannot fulfill the performance metrics in the sixth generation wireless communication systems (6G) [2], i.e., super-high reliability, extreme-low latency and ultra-high transmission rate. Fortunately, space-air-ground (SAG) [3] integrated network will provide global coverage, large bandwidth and flexible deployment functionalities for multiple ground users (GUs). Specifically, SAG is composed of three segments, where the space region [4] can provide global coverage, relay transmission capability and powerful cloud processing for GUs. The aerial region [5] is composed of multiple high altitude platforms (HAPs) loaded with multi-access edge computing (MEC) servers [6], which can supply moderate computation, communication and privacy protection services to GUs. Moreover, HAPs can be flexibly deployed to remote areas or emergencies for the sake of maintaining the basic mobile communication services. Furthermore, the ground region [7] contains massive GUs, whose tasks can be scheduled in the HAPs to satisfy the computation offloading and privacy protection [8].